High power electron gun with electron bombarded apertured cathode having a concave emission surface



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s. SCHILLER ETAL 3,406,305 HIGH POWER ELECTRON GUN WITH ELECTRON BOMBARDED APERTURED CATHODE HAVING A CONCAVE EMISSION SURFACE Filed Oct. 6, 1965 United States Patent 3,406,305 HIGH POWER ELECTRON GUN WITH ELECTRON BOMBARDED APERTURED CATHODE HAVING A 'CONCAVE EMISSION SURFACE Siegfried Schiller and Peter Lenk, Dresden, Germany assignors to VEB Lokomotivbau-Elektrotechnisch Werke Hans Beimler, Henningsdorf, Kreis Oranienburg, Germany Filed Oct. 6, 1965, Ser. No. 493,440

4 Claims. (Cl. 3

ABSTRACT OF THE DISCLOSURE The invention relates to an electron gun in which a cathode having a curved emission surface with a central aperture therethrough produces an axially symmetrical electron beam. Spaced from the cathode an anode is disposed with a transit aperture therein. The inner diameter of the annular cathode is defined by the ratio ml) in which b is the distance from the periphery of the transit aperture of the anode to the periphery of the inner diameter of the cathode and a is the distance from the same periphery of the transit aperture to the outer diameter of said annular cathode, such ratio being preferably in the range of 1.2 to 1.8.

The invention relates to a device for producing an axially symmetrical beam of high power and high perveance with the help of a circularly shaped emission cathode for use in connection with melting ovens, welding or evaporating and material treating apparatus employing such beam, in which device the emission cathode is indirectly heated by means of electron bombardment.

There are electron guns known, in which the electron 5 beam is produced in the same chamber as the object being melted or evaporated. The cathode of these electron guns is directly heated and is in the form of a toroid or a strip which is surrounded by a correspondingly formed anode. The cathode is disposed in close proximity to the object being melted or evaporated. A special embodiment of this gun is known as the transverse gun, in which the focusing and accelerating electrodes are arranged so that they are substantially protected from direct sprays of the flowing metal, so that in this electron gun the electron beam must undergo a 180 shift prior to impinging on the object being melted or welded. Another variation of this electron gun, which is preferably used for melting purposes, is constructed in such a manner that the annular shape cathode thereof surrounnds the object being melted which simultaneously serves as an anode by virtue of its electrical connection. Such annular cathode arrangement has a great disadvantage in that the cathode is exposed to hot metal sprays which enter into alloy combinations with the cathode material resulting in a reduction of the useful lifetime of the cathode.

Furthermore, there are electron guns known which are disposed in a spaced separated from the operational chamber, and which space is individually evacuated. The electron beam of these type of electron gun's enters the operational chamber through flow impedances and associated pressure stages. This arrangement provides the possibility that the pressure in the space where the electron gun is located may be controlled completely independently from the pressure existing in the melting or operational chamber. It has been found that the melting or the desired process may be carried out at relatively high pressures such as 10- to 10" Torr.

It is also known to separate the chamber where the electron beam is produced from the melting chamber by means of several individually evacuated intermediate chambers, so that a pressure drop ranging in the ratio of Patented Oct. 15, 1968 1:100 can be created between the beam producing chamber (where the electron gun is located) and the melting chamber. The electron source in these electron beam producing arrangements is exclusively an axial and rotation symmetrical two pole system (axial gun) having a focusing electrode which is biased to the cathode potential. Also magnetic lenses are used to guide the beam through the relatively narrow flow impedances which create convenient control conditions for controlling the beam diameter in the operating chamber. Magnetic deflector means secure the desired energy distribution on the object being melted. The axial gun employs a solid cathode which is indirectly heated by electron bombardment. By imparting a certain curvature to the emission surface and by means of a correspondingly shaped focusing electrode and anode, a prefocusing of the electron beam is accomplished, which beam otherwise would be strongly divergent due to space charge etfects'and the dispersing effect of the magnetic lenses. Such electron guns have been designed to operate at a power out-put of 200 kw.

Recent developments in the field of elect on beam melting require, however, higher beam outpuis so that ingots, having large dimensions and which require relatively low remelting energy per kilogram of their weight, could be produced.

An increase of the output of axial guns beyond 200 kw. encounters difllculties which have been considered hitherto as unsurmounta-ble. Such difficulties; for instance, arise in connection with the necessity of increasing the accelerating potential to about 35 kv., thereby creating design difiiculties with regard to the shielding of X-ray radiation. In addition to this, the possibility of high voltage flashovers increases with an increase of the accelerating potential. On the other hand, an increase of the current magnitude carried by the electron beam cannot simply be accomplished by increasing the emission surface of the cathode, since this would necessarily require a corresponding increase of the anode aperture. An increase of the anode aperture causes a substantial shift of the effective electric field strength away from the center of the emission cathode. Under such circumstances, only the border regions of the emission cathode contribute substantially to the beam current and a highly loaded space charge region is created in front of the cathode surface at the center thereof. These effects influence the focusing to such a great extent as has been shown by experiments that any effort to direct the beam by means of magnetic lenses becomes extremely diflicult.

Efforts have been made in the past to combine separate axial guns having the power output of kw. each into one equipment in order to obtain a higher resulting output. Such equipment, however, has the disadvantage of having a complicated structure, necessarily accompanied with complex operating conditions. The experts of the field remained of the opinion that one single electron gun should be created which alone is capable of producing a power output above 500 kw.

It has been also proposed to produce strip shaped electron beams. For this purpose emission cathodes have been designed which correspond in shape to a strip cut out from the surface of a spherical segment and which appears as a rectangle when viewed from above. The length of the strips would exceed their width. With this design a practically unlimited emission surface has been created which offers a good penetration for the accelerating field over the entire surface of the cathode, since the transit apertures of the focusing and accelerating electrodes are correlated with the shape of the cathode.

It has been also proposed to combine several of these rectangularly shaped cathodes by arranging them on an arc-shaped support and correspondingly correlating the ice I focusing and accelerating electrodes, so that the beam, after having passed the first magnetic lenses, assumes a uniform band-shaped configuration.

Such arrangement has the disadvantage however that the guidance of the electron beam of such high power through the flow impedances disposed between the in dividual pressure stages imposes extreme requirements on the magnetic lenses, such as rotation free lenses should be used.

Another arrangement is further known in which the cathode is a directly heated wire ring, which is surrounded by a focusing electrode having an annular transit aperture. The transit aperture of the anode which is grounded in the mentioned arrangement, is also annularly shaped. The grounded middle portion of the anode in this arrangement is returned to and supported by a supporting system lying at the negative side of the high potential and should be Water cooled. This results in an immensely enlarged cathode diameter, an increased beam diameter which necessarily increases the size of the entire electron gun to such an extent that use of vacuum techniques for decoupling the beam producing chamber from the operating chamber, becomes practically impossible.

Therefore, an object of the invention is to provide a system capable of producing a high power axially symmetrical electron beam above 500 kw.) for use preferably with electron beam melting or similar equipment, in which the operating and beam producing chambers are decoupled from each other by means of vacuum techniques, and in which a substantially flat circularly shaped cathode is used which is capable of emitting a rotationally symmetrical electron beam adapted to be focused by known magnetic lenses.

Another object of the invention is to eliminate any space charge effects in front of the cathode center.

In accordance with the invention, a mass or emission cathode indirectly heated by electron bombardment has a curved emission surface, which surface preferably assumes a cup shape, the radius of curvature of the cup shaped surface being correlated with the average distance of the cathode from the anode, the distance of the cathode from the focusing electrode and with the diameter of the circular transit aperture provided in the focusing electrode for the electron beam.

In the arrangement in accordance with the invention a relation is created between the power output of the electron beam producing system and the radius of curvature of the emission surface.

The invention also provides the possibility that the emission surface may take forms other than the mentoined cup-shaped embodiment. The border regions of the emission surface, for example, may have a stronger curvature than the inner regions, so that the curved surface in its cross section assumes the outline of a basket. In accordance with the invention, the emission cathode has formed in the center portion thereof an aperture. This aperture preferably is a bore. The dimensions of this aperture are calculated from the ratio of the distance between the edge portion of the transit aperture in the anode at the end thereof adjacent to the cathode, and an inner edge of the annular emission surface of the mass cathode; to the distance between the same edge portion of the anode and an outer edge of the effective annular emission surface of the mass cathode. This ratio should remain always less than two. In addition to the annular emission surface of the mass cathode, a circular transit aperture is provided for the electron beam in the anode as well as in the focus-: ing electrode, the latter being biased to the cathode potential as in the case of cathodes which do not possess a center aperture.

The side of the mass cathode which is opposite to its emission surface represents a substantially flat annular surface. On this opposite surface there is disposed a di rectly heated cathode which is preferably star-shaped. The electrons emitted by this cathode heat the mass cathode.

Positively charged ions which are formed from the residual gas are attracted by the mass cathode and pass through the aperture in the mass cathode as well as through the center opening of the star-shaped, directly heated cathode toward an ion trapping means disposed beyond the star-shaped cathode.

In order to avoid mechanical deformations and misalignments of the electron gun due to the high power outputs and the accompanying heat generation, the mass cathode is supported by three supporting means arranged circumferentially with an angular displacement of between each, the supporting means having high heat resistant characteristics and being connected to the supporting means of the electron gun.

The directly heated star-shaped cathode is supported at the input terminals and at a further point which is electrically isolated from the rest of the circuit.

The technical and economical advantages and the tech nical progress of the invention reside in the fact that the practice of the invention permits the production of an electron beam having a power output of substantially more than one megawatt and in that the beam is produced by a single axially symmetrical electron gun, the diameter of which remains at a small value permitting the use of a greater number of flow impedances between the individual vacuum stages along the entire beam path, the length of the vacuum stages being relatively large with respect to their diameter so that a sufiiciently large pressure drop can be created between the beam producing chamber and the operating chamber.

The arrangement, in accordance with the invention, provides a practically uniformly centered electron beam which does not have in the center thereof, within the accelerating space, a space charge region which is a common disadvantage of the prior devices causing defoousing.

In addition to the above advantages, there is also obtained a favorable relation between the energy input and the useful output. The invention has the further advantage that the ions formed from the residual gases and creating a counter flow with respect to the electron beam, do not lead to a cathode spattering which would lead to a limited lifetime of the emission cathode. The ions pass undisturbed through the aperture in the emission cathode towards the ion trap means since, it has been found that, the diameter of the ion beam is less than the diameter of the aperture in the emission cathode. One of the greatest achievements accomplished by the invention is that despite the presence of a high accelerating potential, the number of high voltage flashovers and corona discharges will diminish, due to the fact that no ions and vapors will be produced by cathode spattering. This advantage will reflect advantageously in the achieved operational safety of the high voltage equipment. This circumstance makes it possible that higher accelerating potentials can be used, by practicing the in vention, than ever before.

The invention will be described with reference to a preferred embodiment thereof shown, by way of example, in the accompanying drawings, in which:

FIG. 1 is a view in section of an electron beam producing system in accordance with the invention, in a simplified form; and

FIG. 2 is a plan view of the directly heated cathode disposed in space relation to the emission cathode.

FIG. 1 of the drawings shows an electron gun of the beam producing system having an electron beam 1 emitted by an emission or mass cathode 2. The mass cathode 2 comprises a cylindrical body the lower side of which, as seen in the drawing, comprises an emission surface 3. The emission surface 3 is formed into a cup shape, the radius of curvature of which depends from the diameter of the mass cathode 2 and the form and the diameter of the transit apertures formed in the adjacent focusing electrode 4 and anode 5. In the center of mass cathode 2 there is formed an aperture 6. The dimensions of this aperture have been calculated from.

the ratio of a distance designated by b representing the distance between anode edge 5 to the inner edge of the effective emission surface 3, to a distance designated by a representing the distance between anode edge 5 to the outer edge of the effective emission surface 3.

The ratio .between distances b and a should always be kept smaller than 2. Especially good results can be obtained with ratios in the range of 1.2 to 1.8. The dimensions of the aperture 6 may also be calculated from the effective emission surface necessary to produce a particular beam.

The aperture 6 itself may have other appearances than the bore shown in the preferred embodiment. -It is, for example, possible to have a bore which is tapered toward the surface opposite to the emission surface of the cathode, the tapering being etfected in several stages. It may also assume the form of a truncated cone.

The surface of the mass cathode 2 lying opposite to the emission surface 3 thereof has the form of an annular body 7 (see FIG. 2). The mass cathode 2 is usually made of tungsten, tantalum, lanthanhexaboride (LaB or similar materials.

On the annular surface 7 a directly heated cathode 8 is disposed. The directly heated cathode 8 is prefer-= ably st-ar-shaped which form facilitates a better conversion of the input energy since only the annular surface 7 need be bombarded by electrons, The mass cathode 2 is heated by the impinging electrons emitted by the directly heated cathode 8. With this arrangement, the aperture 6 of the emission cathode 2 remains free also in the region of the directly heated cathode 8. The invention provides also a relatively simple support for directly heated cathode 8. Cathode 8 is supported on the input terminals 9, 10, and at a further point 11 which is electrically isolated from the rest of the circuit. It is also possible to provide other supporting points, as the high temperatures may require. It has been found that the secondary support at point 11 is sufficient.

The mass cathode 2 is supported by at least three substantially rectangularly shaped brackets 12 arranged circumferentially with an angular displacement of 120 between each. Brackets 12 have high heat resistant ch-ar-= acteristics and are joined with supporting means 13 of the electron gun. Brackets 12 are preferably made of tungsten, which during expansion of the mass cathode under the influence of heat will exhibit a compensating effect, so that the dimensional variations produced by the high emission temperatures will not exceed in the order of about one millimeter, which will not lead to eccentricity of the mass cathode 2. Brackets 12 are spot welded to the supporting means 13 which a e made of. molybdenum rods, which construction permits temperatures jumps of the order of between -800 and 1000" C. Brackets 12 also provide good heat conduction. Such construction leads to only a slight heating of the po-rtions of the electron gun which are not water cooled, whereby only very slight movement of the entire electron beam producing system is experienced.

The arrangement in accordance with the invention leads to the accomplishment that a dense, prefocused rotationally symmetrical electron beam 1 is present in the region adjacent the transit aperture of anode 5, which beam is easily adapted to further focusing and guidance through the system.

The positive ions 14 formed from the residual gases tend to flow toward the mass cathode 2 and pass through aperture 6 and through the center of the star-shaped cathode 8 without causing any disturbance and reach the ion trapping means 16 disposed in a shielding cylinder 15, the ion trapping means having a cone-shaped recess 17 formed therein. v

It is also possible that the emission surface 3 of the solid cathode 2 will possess a formdiiferent from the discussed cup-shaped form. The emission surface 3 should be correlated with the shape of the focusing electrode 4 and that of the anode 5 in order to obtain better prefocusing conditions. Such variation of the emission surface 3 may involve a more pronounced curvature of the edge zones thereof when compared to the curvature of the middle regions. In cross-section such variation would assume a basket outline as shown with dotted lines in FIG. 1.

Although the invention has been described with refer ence to a specific embodiment thereof, it is not intended that the invention should be limited to the disclosed specific embodiment but defined by the scope of the appended claims.

What is claimed is:

1. Device for producing an axially symmetrical electron beam comprising: anode means having a transit aperture therethrough for said beam; indirectly heated cathode means disposed axially spaced from said anode means, said cathode means comprising a curved emission surface and having in the center portion thereof an aperture of predetermined diameter extending therethrough, whereby said emission surface is generally an= nular in shape, wherein said diameter is determined by the ratio of the distance between an edge portion of said transit aperture at the end thereof adjacent said cathode means and an inner edge portion of said emis sion surface; to the distance between said edge portion of said transit aperture and the end thereof adjacent said cathode means and an outer edge portion of said emission surface, said ratio being less than 2, wherein directly heated cathode means are disposed on the side of said indirectly heated cathode means opposite said emission surface thereof.

2. Device as claimed'in claim 1, wherein said directly heated cathode meansis star-shaped.

3. Device as claimed in claim 2, wherein said directly heated cathode means comprises input terminal means, said directly heated cathode means being supported on said input terminal means and mat least one support point electrically isolated from saidterminal means.

4. Device as claimed in claim 1, wherein ion trap means are disposed axially of said device and in spaced relationship with said directly heated cathode means.

References Cited UNITED STATES PATENTS 2,362,937 11/1944 Shepherd 313-339 2,684,453 7/1954 Hansell 313-339 2,760,097 8/1956 Eber et a1. 313-7l 3,132,275 5/1964 Merdinian 313-82 JAMES W. LAWRENCE Primary Examiner.

V. LAFRANCHI, Assistant Examiner. 

